Synapses are highly specialized dynamic structures, site of neurotransmitter release and postsynaptic responses. Synaptic dysfunction has been implicated in a number of developmental, psychiatric, and neurodegenerative disorders. Cdk5 is a kinase essential for brain development and implicated in synaptic function and neurodegenerative disorders. Our studies indicate that cdk5 phosphorylates the postsynaptic protein PSD-95 in vivo. PSD-95 is a scaffolding protein implicated in the stabilization and signaling of glutamate receptor at synapses. Since cdk5-dependent phosphorylation of PSD-95 decreases PSD-95 association with membranes and clustering of PSD-95, we propose to test the hypothesis that phosphorylation negatively affects the intracellular trafficking of PSD-95 and its ability to stabilize glutamate receptors at synapses. Furthermore, we will test the possibility that cdk5 activity and cdk5-dependent phosphorylation of PSD-95 are regulated by glutamate receptors activity. The specific aims of this proposal are: 1) To characterize the biochemical properties of phosphorylated PSD-95; 2) To elucidate the regulation of cdk5-dependent phosphorylation of PSD-95; 3) To assess the impact of phosphorylated PSD-95 on trafficking of PSD-95 and glutamate receptors. This novel research proposal will use established biochemical, immmunocytochemical, and imaging methods to address the impact of the cdk5-dependent phosphorylation on synaptic organization and function. These studies will provide new insight into the molecular organization and regulation of synapses, and the molecular mechanisms responsible for the formation of brain circuitry, learning, and memory. The long-term goal of this project is to further our understanding of the function of cdk5 at synapses and to identify molecular mechanisms relevant to synapse function. These results will be relevant to neurodegenerative and psychiatric disorders associated with synaptic dysfunction, such as Alzheimer's, Huntington's, and schizophrenia.